1. Field of the Invention
The invention relates to a gastight bottom construction for a shaft furnace, having a plurality of horizontal layers of refractory material and a gastight metal membrane above at least the lowermost of the refractory material layers. The membrane is sealed to the furnace shell.
2. Description of the Prior Art
A bottom construction is disclosed in U.S. Pat. No. 4,157,815 in which a metal membrane or separating layer is described in combination with a cooling system in the bottom formed by tubes through which coolant flows. These tubes and the metal membrane are located in a rammed layer of refractory material. The metal membrane serves to prevent gas leakage into the cooling system and liquid leakage. from the cooling system to the inside of the furnace. The metal membrane consists of a single layer of copper plate or a plate of some other metal which is formed by brazing together strips and brazing edging pieces to the steel shell of the furnace to give a single coherent layer.
As a rule metallurgical shaft furnaces, and blast furnaces in particular, are constructed without means of cooling in the bottom, the bottom construction of refractory material being directly built up on a supporting structure of non-refractory bricks or concrete within an approximately cylindrical wall construction. If the furnace is operated under raised gas pressure, this type of bottom construction can permit gas to leak away downwards and escape under the bottom edge of the steel shell. This can be dangerous, and from the operating point of view is undesirable. The need has therefore arisen for a gastight bottom construction for a furnace which is not provided with cooling means in the bottom and which therefore may not have a rammed layer of refractory material.
In addition, there are disadvantages of a gastight metal membrane consisting of one piece, which may for example be made by brazing strips together. In the first place the fabrication of such a membrane and its attachment to the steel shell is very labour-intensive and requires precise control. Furthermore, during the heating of the furnace from cold the thermal expansion of the shell and of the refractory construction of the bottom, as a whole and in its component parts, can give rise to high stresses in the metal membrane, with very high local stress concentrations. The consequence of this can be failure of the gastightness of the membrane. In the case of a bottom construction which includes layers of refractory blocks it is very undesirable to have a metal membrane of plates or strips brazed together. For example, the local thicker parts which result from the brazed seams impede the stacking of a compact flat layer of refractory blocks above the membrane. Because the blocks do not lie properly on the membrane, a very irregular thermal contact pattern is produced, which can cause undesirable temperature differences throughout the bottom construction.